Field of the Invention
The present invention relates to a method of producing circuit boards by photo-etching method of a base material provided on at least one side with the metal foil which is subsequently etched to provide a conductive pattern.
Prior Art
It has been found desirable to have circuit boards which have a circuit pattern on a surface which pattern is formed of a conductive metal layer with a thin corrosion-resistant layer on an exposed surface thereof. To produce these circuit boards, the circuit board with an exposed metal layer of the conductive metal is printed with a galvo-resistant negative image of the required conductor pattern. The conductor image formed by the negative image is galvanically reinforced with a thin, corrosion-resistant and etch-resistant metal layer. After the negative image is removed, the regions of the metal foil which do not correspond to the conductor pattern are removed by etching. During the etching process, more metal is removed at the edge of the conductor pattern beneath the etch-resistant metal layer than in the direct vicinity of the base material. This under-etching or under-cutting occurs at the edges of the conductor pattern immediately beneath the etch-resistant metal layer due to a longer exposure of this area to the etching agent. Furthermore, the contact between the two different metals of the two layers may accelerate the removal of the conductive metal layer in the area immediately adjacent to the corrosion-resistant layer.
The under-etching or under-cutting of the corrosion-resistant layer generally leads to breaking off of the overhanging projections of the corrosion-resistant layer from the condutor pattern. This not only results in a loss of accuracy of the contour and shape of the pattern but mainly has the danger that the broken off pieces will form short circuits in the conductive pattern of the circuit board.
Several attempts have been made to reduce the extent of the under-etching or under-cutting of the corrosion-resistant layer. In one attempt, which is disclosed in German Auslegeschrift No. 2,061,991, the etch-resistant and non-corrosive metal layer of the conductor pattern prior to the step of etching is covered with an additional etch-resistant layer. This procedure merely reduces the problems associated with the under-cutting or under-etching, but does not completely prevent the under-etching or under-cutting and therefore does not prevent the occurrence of the above-mentioned disadvantages and problems.